Electromagnetic devices store energy in magnetic fields and recover that energy in order to convert an input of one form into an output of another form. For example, generators convert a mechanical input into an electrical output by using the mechanical input to rotate a magnetic rotor (or field) for storing the energy of the mechanical input in a varying electromagnetic field. A stator winding typically wound around the stator of the generator converts the variable electromagnetic field into electrical energy which is provided at the output of the generator.
A motor, on the other hand, uses an electrical input for storing energy in an electromagnetic field. The energy in that field is then converted by an armature into an output mechanical energy.
A transformer receives electrical energy at a primary winding in order to induce an electromagnetic field. This electromagnetic field is converted by a secondary winding back into electrical energy which is then provided at an output of the transformer. The output voltage (or current) of the transformer may be greater than, equal to, or less than the input electrical voltage (or current).
All of these electromagnetic devices generally incorporate a ferromagnetic core in order to provide a flux path for the electromagnetic fields generated by windings of electrically insulated, current carrying conductors. The core may be a single continuous core member or may be divided into one or more stator cores and one or more rotor cores. Furthermore, all of these cores are typically constructed of a plurality of laminations held or fastened together in a configuration designed to provide the desired flux path.
In designing electromagnetic devices such as generators, motors and transformers, the maximum output of these devices is limited by the maximum amount of heat which such devices can withstand. Such heat is principally produced by losses of the devices themselves. Such losses include copper losses generally referred to as I.sup.2 R losses produced by the resistances of the current carrying conductors in the device, losses produced in slip rings and commutators, mechanical losses produced by brush and bearing friction, windage, and core losses produced by hysteresis and eddy current losses arising from changing flux densities in the iron of the cores of the devices. There may also be stray load losses which include the losses arising from nonuniform current distribution in the current carrying conductors and additional core losses produced in the iron of the core caused by the distortion of the magnetic flux by the load current.
Heat produced by such losses can unduly shorten the life of a machine. The operating temperature of an electromagnetic device is closely associated with its life expectancy because deterioration of the insulation of the current carrying conductors and of the core itself is a function of both time and temperature. Such deterioration is, at least in part, a chemical phenomenon involving slow oxidation and brittle hardening which leads to loss of mechanical durability and dielectric strength.
As a result, electromagnetic devices are often provided with some form of cooling in order to eliminate or reduce such deterioration.
The amount of cooling necessary for an electromagnetic device, in general, increases with increasing size of the device itself. Thus, while the surface area of the electromagnetic device from which heat can be carried away increases roughly as the square of the dimensions of the device, the heat developed by the above-noted losses is, on the other hand, roughly proportional to the volume of the device itself and, therefore, increases approximately as the cube of those dimensions.
In addition to being cooled, a core, particularly a stator, must be retained within a housing so that it remains fixed within the housing and so that the difference in thermal growth between the core and its housing does not damage the housing and/or core in the presence of heat. Also, for many applications, the housing of a stator must be sufficiently strong to act as a barrier for the containment of high energy rotor particles which may be produced by fragmentation of the rotor during rotor dynamic failure. For example, generators, which produce electrical power on aircraft, typically include a hollow stator surrounding a rotor. Because fragmentation of the rotor can be dangerous to the aircraft if the rotor particles, which result from a dynamic failure of the rotor, breach the housing of the generator, the housing of the generator must be made strong enough to prevent such breach.
The cross-section of a portion of a typical electromagnetic device 10 is shown in FIG. 1. The device 10 has a housing 12 and a core 14. The core 14, which may be a stator core, is attached to the housing 12 by a plurality of bolts 16. The core 14 is generally comprised of a plurality of concentric laminations all of which are stacked together and mounted within the housing 12 concentrically to an axis 18. The core 14 has an outer perimeter in the form of a circumference 20 and an inner perimeter in the form of a circumference 22. The circumference 22 forms a cylindrical void 24 into which may be mounted, for example, a rotor. The housing 12 has an inner perimeter 26. The outer perimeter 20 of the core 14 may be, if desired, substantially equal to the inner perimeter 26 of the housing 12. If the void 24 of the core 14 contains a rotor, the combination of the housing 12 and the stator core 14 should have sufficient strength to contain high energy rotor particles produced by a dynamic rotor failure (i.e. a failure of the rotor occurring during rotation of the rotor).
In some constructions, cooling passages can be provided in either the housing 12 or in a sleeve, such as an aluminum sleeve, between the housing 12 and the core 14. Cooling fluid is circulated through these cooling passages in order to draw heat away from the core 14. The bolts 16, which penetrate the housing 12 and the core 14, can cause undesirable leaks of cooling fluid if the bolts 16 should also penetrate the cooling passages. Moreover, the thermal growth rate of the housing 12, which is typically made of a magnesium alloy, is considerably different than the thermal growth rate of the core 14, which is typically constructed from laminations of magnetic iron. Thus, when losses in the electromagnetic device generate heat, deflection of the housing 12 as shown in FIG. 2 is produced. This deflection, if severe enough, can cause structural damage to the core 14 and/or housing 12. Furthermore, the additional stress at the points where the bolts 16 penetrate the housing 12 can facilitate an increase in leakage at those points. Also, if the combination of the housing 12 and the stator core 14 does not have sufficient strength, it will not contain high energy rotor particles produced by a dynamic rotor failure.